To facilitate diesel engine starting, particularly with cold ambient temperatures, the combustion chamber of each of the engine cylinders is generally preheated by an electrically energized glow plug adapted to be threaded into the engine block in communication with the combustion chamber. Upon the electrical energization thereof, the temperature of each of the glow plug heater elements is raised to preheat the corresponding combustion chamber prior to engine "crank". The period of time a glow plug heater element is energized prior to engine crank, the preheat period, is determined by engine temperature and glow plug heater element energization potential magnitude (i.e. the lower the temperature and/or the lower the glow plug heater element energization potential magnitude, the longer the preheat period required).
With many glow plug energization control systems, the glow plug heater elements are energized at rated energizing potential. Although this rated glow plug heater element energization potential prevents premature failure as a result of overheating, the period of preheat before engine crank may be of the order of one or two minutes or longer with colder ambient temperatures. To substantially reduce the preheat period the glow plug heater elements may be energized at greater the rate of energizing potential. With glow plug heater element energization at greater than rated potential, however, it is necessary that the heater elements be cyclically energized for successive periods of time just long enough to increase the temperature thereof to a predetermined maximum. Systems of this type are disclosed and described in U.S. Pat. Nos. 4,137,885 and 4,177,785. Other glow plug energization systems, generally of the type noted above, are disclosed in U.S. Pat. Nos. 4,088,109, 4,307,688, 4,444,160 and 4,452,191.
As noted in U.S. Pat. No. 4,726,333 in order to ignite an alternative fuel, such as alcohol, in a diesel engine, the fuel must be heated before or during compression. Glow plugs conveniently provide heat, but have a relatively short life in continuous applications. Such glow plugs must be accurately controlled to prevent overheating, which leads to open circuiting.
U.S. Pat. No. 4,512,295 discloses a system for protecting against the enlargement of diesel glow plug tips. Separate protective devices are connected in series to its individual glow plugs, which open in response to current through the protective device to a value corresponding to a short of the heating element in the tip of the glow plug.
U.S. Pat. No. 4,487,169 discloses an electronic control unit for an internal combustion engine wherein a duty cycle rate generator alternately connects and disconnects a source of DC voltage to the glow plugs above the present value at a duty cycle rate which ranges from 0 to 100% and which depends on the voltage level of the source of DC voltage.
One approach to control glow plug energization is to turn the glow plugs on and off through the use of a electromechanical relay. The number of on/off cycles required in an operation such as an urban bus operation, however, can quickly exceed the life of the relay so that periodic expensive replacement is required.
Consequently, there is a need for a method and system for controlling the energization of at least one glow plug to provide fast warmup while minimizing the chance of an overtemperature condition for the glow plug. Also, there is a need to have a method and system to control the energization of the glow plug, which is not only reliable, but also ensures a long life for the system which controls the energization of the glow plug.